1. Field of the Invention
The invention relates to mobile communication technology, more particularly to the handover method of mobile communication, applicable for designing open-loop or closed-loop synchronization technology for digital mobile communication systems.
2. Related Technology
To guarantee continuity of communication service when a mobile terminal is moving or the location of the mobile terminal is changing, all wireless communication systems adopt handover technology, which includes soft-handover, hard-handover, and handover baton handover, etc.
FIG. 1 briefly illustrates flow of the current handover modes. (1) Assuming that a wireless link service connection is already established between a mobile terminal and a base station 1, and communication is maintained; (2) the mobile terminal measures the wireless signal from an adjacent cell, mainly measuring power, distance; and voice quality since these three values determine the handover threshold, then compares the handover threshold thereof with a handover decision criterion established in the mobile terminal; if the handover threshold thereof exceeds the decision threshold, the mobile terminal reports the measurement report containing a handover request and measuring results to the network; the network integrates the reported power, distance and voice quality, and calculates a handover threshold, then compares this handover threshold with the handover decision threshold stored in the network, performing the handover decision and determining whether to switch; (3) when deciding to perform handover, the network a sends handover command to the mobile terminal and switches the mobile terminal to a base station 2 with a prepared wireless link; (4) wireless link synchronization is established between the mobile terminal and a target base station 2; (5) wireless link service connection is established between the mobile terminal and the handover target base station 2; and (6) after the mobile terminal transmits confirmative information of handover success, the wireless link between the mobile terminal and the base station 1 is released.
In most CDMA systems, soft-handover or hard-handover is adopted, whose characteristics are: before handover, besides maintaining communication with the handover-forward base station (i.e. base station 1), the mobile terminal has to measure signals of adjacent cells; during the handover procedure, there is a synchronization-establishing procedure between the mobile terminal and the target base station (i.e. base station 2).
For the existing handover of mobile communication systems, when the base stations are already synchronous or synchronization variation is known and constant, the uplink synchronization time can be precisely calculated according to the measured values of base station downlink signals measured by the mobile terminal. If synchronization is not implemented among the base stations, in most cases, wireless link synchronization between the mobile terminal and the target base station will not be established until not only the to-be-switched-to target base station is determined but also the network decides to perform handover, so the synchronization procedure occupies the time of handover. When the mobile terminal is in a fast-moving state or a continuous location-changing state, excessive handover time occupation will inevitably adversely affect the handover success rate and reliability thereof, and decrease handover quality.